Anesthesia for Posterior Fossa Surgery

🔹 Introduction

Posterior fossa surgeries involve the cerebellum, brainstem, and fourth ventricle — areas that are densely packed and functionally critical.

These surgeries pose unique anesthetic challenges due to:

  • Close proximity to brainstem and cranial nerves
  • Obstructed CSF pathways
  • Positioning risks (especially sitting position)
  • Risk of hemodynamic and respiratory instability


🔹 Common Indications

  • Cerebellar tumors (e.g., medulloblastoma, hemangioblastoma)
  • Brainstem gliomas
  • Acoustic neuromas / Vestibular schwannomas
  • Chiari malformation
  • Fourth ventricle tumors or cysts
  • Posterior fossa arteriovenous malformations (AVMs)


🔹 Surgical Approaches

  • Suboccipital / Retrosigmoid craniotomy
  • Midline / Transvermian approach
  • Far-lateral approach
  • Sitting position approach

Each approach dictates patient positioning, which influences:

  • Airway access
  • Hemodynamics
  • VAE risk
  • Neurophysiologic monitoring


🔹 Preoperative Considerations

1. Neurological Assessment

  • Evaluate cranial nerve deficits (CNs V–XII)
  • Signs of raised ICP or hydrocephalus
  • Ataxia, nystagmus, dysphagia, respiratory irregularity

2. Airway

  • Anticipate difficult intubation if:
    • Craniovertebral anomalies (e.g., Chiari)
    • Prior surgeries/radiotherapy
  • Evaluate for bulbar dysfunction (aspiration risk)

3. Imaging

  • Assess brainstem involvement, hydrocephalus, and vascular structures


🔹 Positioning and Related Concerns

1. Sitting Position

  • Advantage: Surgical access, drainage, visibility
  • Disadvantages:
    • Venous air embolism (VAE) risk (up to 45%)
    • Hypotension, bradycardia, due to venous pooling
    • Paradoxical air embolism (if PFO present)
    • Pneumocephalus

2. Lateral / Park Bench Position

  • Common in acoustic neuroma resection
  • Risks: Shoulder/nerve injuries, dependent lung hypoventilation

3. Prone / Concorde Position

  • Used in midline suboccipital approaches
  • Risks: Abdominal compression, airway difficulty, ocular pressure


🔹 Anesthetic Management

🔸 Induction

  • Smooth, controlled to avoid ICP surges
  • Use propofol, opioids, non-depolarizing NMBs
  • Secure airway with reinforced ETT (especially in prone/sitting)

🔸 Monitoring

Monitoring

Purpose

Invasive BP

Beat-to-beat control

Central Line

Air aspiration, CVP

Precordial Doppler

Detect VAE

EtCO₂

Sudden ↓ = VAE

BIS / Entropy

Depth monitoring

NIRS

Cerebral oxygenation

EEG / Evoked Potentials

CN and brainstem monitoring


🔸 Maintenance

  • TIVA (propofol + remifentanil) is preferred for neurophysiologic monitoring
  • If inhalation used: <1 MAC of sevoflurane
  • Avoid N₂O — worsens pneumocephalus, ↑ VAE risk
  • Ensure mild hyperventilation (PaCO₂ ~30–35 mmHg)
  • Maintain normothermia, euvolemia


🔹 Venous Air Embolism (VAE)

🩸 Signs of VAE

  • Sudden ↓ EtCO₂
  • Mill wheel murmur (precordial Doppler)
  • ↑ CVP, ↓ BP, ↓ SpO₂

🛠️ Management

  • Flood field with saline
  • Lower head temporarily
  • Aspirate air via central line
  • 100% O₂, stop N₂O (if used)
  • Trendelenburg, left lateral (Durant’s maneuver)
  • CPR if cardiovascular collapse

🧠 Always rule out PFO preoperatively if considering sitting position (via TEE or contrast echo)


🔹 Intraoperative Challenges

Challenge

Management Strategy

Brainstem manipulation

Risk of bradycardia, apnea → Prepare glycopyrrolate/atropine

CN monitoring

Avoid long-acting NMBs; use TOF

Brain bulge

Mannitol, hyperventilation

CSF obstruction

May need ventriculostomy

Swallowing dysfunction

Consider NGT, delay oral intake postop

Respiratory irregularity

Anticipate prolonged ventilation



🔹 Postoperative Considerations

  • Airway edema: Due to prone/sitting positioning, brainstem trauma
    → Extubate only if fully awake, protective reflexes intact
  • Cranial nerve palsies: Hoarseness, aspiration risk
    → Swallowing test, NPO until cleared
  • Respiratory depression: Central (brainstem) or due to opioids
    → Monitor ABG, SpO₂ closely
  • Cerebellar mutism: Especially in pediatric medulloblastoma surgery
  • Tension pneumocephalus: Look for “Mount Fuji sign” on CT if delayed awakening
  • Hypertension: From ICP rebound, pain, or autonomic dysregulation


🧠 Key Viva Points

Q: What is the most feared complication of posterior fossa surgery?
A: Brainstem injury and VAE

Q: Why avoid N₂O in posterior fossa surgery?
A: Increases risk of pneumocephalus and expands air emboli

Q: Which cranial nerves are commonly affected?
A: CNs V–XII (especially IX, X → swallowing issues)

Q: What is cerebellar mutism?
A: Postoperative syndrome in children — speech loss, hypotonia, emotional lability


🔍 References

  1. Miller’s Anesthesia, 9th ed. — Neuroanesthesia and Positioning
  2. Cottrell & Young’s Neuroanesthesia
  3. StatPearls: Posterior Fossa Tumor Resection
  4. BJA Education: Sitting Position in Neurosurgery
  5. WFSA: Neuroanesthesia tutorials